Inverse stereochemical effects in the mass spectra of carboalkoxycyclopropylsilanes

The electron impact induced decomposition of the cis and trans isomeric pairs of five 1-trialkylsilyl-2-alkoxy-carbonylcyclopropanes were investigated. Compared with carbon analogues, the silicon atom inhibited ring opening and dramatically altered the fragmentation. The main primary decomposition routes were alkyl loss from silicon, and trialkylsilyl ion formation as well as trialkylalkoxysilane elimination. An unexpected inverse stereochemical effect was found for this latter reaction which was far less significant for the cis isomers, where the interacting SiQ₃ and OR groups are close to each other, than for the trans isomers. This is explained by assuming a hidden silicon-oxygen bond formation, which is possible only in the cis isomers for geometric reasons. This interaction favours alkyl elimination over other fragmentations, and has a marked effect on the further decomposition of the M—alkyl]⁺ ions, where observed differences in abundance as great as three orders of magnitude are accounted for.